The invention concerns a process and arrangement for the alignment of the image plane of an imaging system with an image interception plane wherein a measuring mark is reproduced by at least two partial pupils of the imaging system and the imaging beams associated with said partial pupils are affected differentially.
2. Description of the Prior Art
Every optical imaging system comprises a certain imaging plane as a function of the distance of the object, wherein the image of the object appears in sharp focus. When the imaging system is a component of an optical device, then the imaging plane of the imaging system must be coordinated with the image interception plane of said device in order to produce a sharp image. Interception planes in the sense used herein are, for example, the film plane or the ground glass plate of a camera, the intermediate image plane or the graticule plane in a microscope, the photocathode in an image converter tube, or the like. The coordination between the image plane and the plane of interception is designated alignment or tuning.
The simplest visual method of alignment consists of producing an image in the intercepting plane desired by means of the imaging system and to observe this image. The aligning criterion herein is the setting of the image displaying the highest contrast or an image with a characteristic color hue. The advantage of this method resides in the fact that the imaging system is being aligned under the identical conditions whereby it is used. A disadvantage, however, is that consistency of the setting is small because it is affected under serial production conditions, by the fatiguing of the eyes, subjectively in contrast evaluation and differential chromatic eye sensitivity.
In another method, the imaging system is made to produce the image of a slit over two small partial pupils in the intercepting plane desired, said partial pupils being located as much as possible at opposing sides of the overall pupil. The image over the two partial pupils consists either of a mutually overlapping mixed image or a complementary total image (vernier setting). The beams coordinated with the partial pupils may be made optically distinguishable by means of color or polarizing filters. The aligning criterion in both cases is represented by the best possible combination of the partial images into a total image, said partial images moving opposedly with respect to each other during the adjustment of the imaging system. This coincidence system lends itself to a more secure evaluation than the sharp focusing mentioned hereinabove. It has, however, the disadvantage that the imaging system is used in a different manner during alignment than in the instrument itself. The alignment therefore leads to uniform results only when the image produced by means of the two partial pupils always maintains the same position in the case of all objective lenses of the same computation with respect to the image produced by the total pupil. This signifies in other words that the sharp focusing and coincidence setting procedures must not produce different results.
In a modified embodiment of the aforementioned procedure, imaging of an object takes place by means of one partial pupil, for example a single slit, and by means of the other partial pupil a double slit. Both beams are deflected by prisms (deflecting wedges) so that the image of the single slit rest inbetween the image of the double slit. The aligning criterion here is given by the symmetrical position of the slit images with respect to each other. This setting may be evaluated with more accuracy than even the coincidence setting. It is essential, however, that the refracting edges of the deflecting wedges be adjusted exactly in relation to the direction of the slits, because otherwise the deflecting wedges would introduce a degree of unsymmetry in the mutual position of the slit images, which would be interpreted as a false setting of the imaging system. Even in the case of an initially correct alignment of the deflecting wedges, there are no means provided to indicate subsequent changes in the setting of the device. Obviously, the disadvantage that the image produced by the partial pupils does not necessarily equal the image created by the total pupil, is again present.
It is therefore the object of the invention to provide a method operating with an alignment criterion which avoids the disadvantages of imaging by means of partial pupils of the imaging system, which further improves the reliability of alignment and which is independent of the setting of the deflecting wedges in symmetry alignments.